A UPS is used to provide a constant source of power to a critical load. A UPS generally receives power at an input port from a utility power source and supplies power from the input port to an output port for critical loads. When the power at the input port from the utility power source is disrupted, even for a brief interval, the UPS provides power to the output port adequate for the demand of the critical loads from a secondary power source such as a battery or capacitor. When the utility power is restored at the input port, the UPS, again, supplies power to the load on the output port from the input port. In addition, an emergency generator may be connected to the input port of the UPS. For example, if the utility power is not restored quickly, an emergency generator may be connected to the input port of the UPS to prevent the secondary power source (battery) from becoming depleted.
Utility companies and customer sites require that, in a high power UPS, the UPS not instantaneously switch the source of power for the load on the output port from the secondary power source to the input port. The output port of a high power UPS may provide, for example, 80 kW (kilowatts) of power for critical loads. This level of instant demand from the utility source may cause damage to the utility's delivery vehicles, e.g., transformers, trip feeder breakers, and could interact with other devices using the input feed to the input port. In addition, if an emergency generator is placed on the input port, the level of demand may trip a breaker on the generator. As a consequence, customer require that the level of power demand on the input port (such as from a utility source or an emergency generator) be increased on a gradual basis. This is known in the art as a "power walk-in".
Known high power UPS's provide a "power walk-in" from the input port by measuring the current level at the input port. The UPS gradually increases the power supplied from the utility source to the load on the output port based on the measured current level at the input port. The power supplied from the input port to the load on the output port, however, varies according to the input voltage, input current and input power factor.
For example, UPS's typically employ a rectifier to control to level of power supplied from the input port to the output port and employ input filters to smooth signals from the input port. The power factor of a rectifier normally changes from no load to full load. The input filters also affect the input power factor. As a consequence, the UPS can not accurately determine and, thus, control, the level of power supplied from the input port by a utility to the load on the output port. Therefore, prior art UPS's can not provide a strictly controlled input power walk-in or "true" input power walk-in.
In addition, prior art input power walk-in circuitry is complicated. In particular, the circuitry, first, includes three current transformers. The three current transformers measure the input current, one for each of the three phases of AC power supplied by the utility. The circuitry further usually includes at least 6 operational amplifiers ("OPAMPS") and 16 resistors and/or capacitors. The OP AMPS, resistors, and capacitors rectify the three measured currents and average the rectified currents together to generate an indication of the input current. This circuitry is complex and expensive. Therefore, a need exists for an input power walk-in process and "power walk-in" circuitry that provides a true "power walk-in" and is less complicated than the prior art.